In recent years, reduction in size and improvement in performance have become increasingly evident in portable information terminals such as personal computers (PCs), cell phones and personal digital assistants (PDAs), and audio visual devices such as video recorders and memory audio players, for which lithium ion secondary batteries are used.
As such reduction in size and improvement in performance continues, the improvement in capacity of lithium ion secondary batteries has been needed. In the conventional lithium ion secondary batteries, a lithium-containing transition metal oxide, such as lithium cobaltate or lithium nickelate, is used for a positive electrode, and a carbonaceous material, such as graphite, is used for a negative electrode. However, the improvement in capacity achieved by a combination of these materials in commercially available lithium ion secondary batteries has been approaching the limit of improvement.
Under these circumstances, examination has been made on selection and design of such a negative electrode active material that enables improvement in capacity in lithium ion secondary batteries to be accomplished. The negative electrode active materials for improvement in capacity that are under examination are exemplified by metal lithium, aluminum to be alloyed with lithium, silicon, tin, and the like (e.g., Non-patent Document 1). Among these, silicon has a large theoretical capacity. For this reason, there has been proposed a lithium ion secondary battery in which silicon is used as an active material (e.g., Patent Document 1).
However, since silicon undergoes a significant volume change during the reaction with lithium ions, the current collecting performance is reduced by repeated charge and discharge, making it impossible to achieve sufficient cycle characteristics.
As a negative electrode active material for solving this problem, there has been proposed SiOx (0<x<2) (e.g., Patent Document 2 and Patent Document 3). The SiOx has a high capacity and exerts stable cycle characteristics. However, Li having been inserted in the SiOx during charge in the early stage is not completely released therefrom, causing a so-called irreversible capacity to increase.
In view of the above, the use of a lithium silicate compound originally containing Li has been proposed. For example, a lithium silicate compound represented by LiySiOx (0<y, and 0<x<2), such as Li4SiO4, Li2SiO3, Li2Si2O5, Li4Si3O8, and Li6Si4O11, has been proposed (Patent Document 2). Moreover, in order to obtain favorable cycle characteristics, there has been proposed a negative electrode active material obtained by heating SiOx to crystallize it so that a diffraction peak corresponding to the (220) plane of Si can appear in the X-ray diffraction pattern (Patent Document 3).
In addition, in order to suppress the volume expansion due to charge and discharge, there has been proposed a negative electrode active material including silicon with an oxidation number of 0, a silicon compound having a silicon atom with an oxidation number of +4, and a lower oxide of silicon having a silicon atom with an oxidation number of greater than 0 and less than +4 (e.g., see Patent Document 4).
Furthermore, there has been proposed a SiO thin film studied with the use of X-ray photoelectron spectroscopy (XPS), the thin film including silicon with an oxidation number of 0, a silicon compound having a silicon atom with an oxidation number of +4, and a lower oxide of silicon having a silicon atom with an oxidation number of greater than 0 and less than +4. The charge-discharge reaction mechanism of the SiO thin film has also been disclosed (e.g., see Non-patent Document 2).
In a silicon oxide having a chemical composition represented by SiO, on a microscopic scale, Si and SiO2 are present in a phase separation state. It is known therefore that SiO represents an average composition (e.g., Non-patent Document 3).
The silicon oxide has structural units having a tetrahedral geometry. Other forms of silicon oxide other than SiO2 (hereinafter referred to as intermediate oxides) can be represented by Si2O, SiO, and Si2O3 in accordance with the number of oxygen atoms on the vertices of the tetrahedron, one, two, and three, respectively. These intermediate oxides are thermodynamically unstable and are extremely difficult to present as a simple crystal. Presumably for this reason, SiO represents an average composition as described above. It should be noted that X-ray photoelectron spectroscopy (XPS) of the intermediate oxides as described above clearly detects two separate peaks attributable to Si and SiO2.
Patent Document 1: Japanese Laid-Open Patent Publication 2002-83594
Patent Document 2: Japanese Laid-Open Patent Publication Hei 6-325765
Patent Document 3: Japanese Laid-Open Patent Publication 2004-71542
Patent Document 4: Japanese Laid-Open Patent Publication 2005-183264
Non-patent Document 1: Solid State Ionics, 57, 113-115 (1998)
Non-patent Document 2: Journal of The Electrochemical Society, 152 (10), A2089 (2005)
Non-patent Document 3: Journal of Non-Crystalline Solids, 204 (2), 202-203 (1996)